After several false starts and aborted attempts I have come to the conclusion that I cannot satisfactorily answer the question. Why does anyone write? What combination of innate talent, social setting, circumstance or one's age results in writing? And when one does write what relationship is there between one's avowed purpose, one's secret purpose, or even some purpose not realized? That the three authors were strongly motivated to write is clear by the fact that they did so and by the evident amount of time and care they put into their writing. That they wished to share what they had written is also clear. But what were their motives and what did they set out to accomplish?
There are certainly some assumptions that can be made in reading these selections and some similarities between them*. Petrarch, though he later repudiates such feelings, certainly expresses his passion in the poems. While the extent, nature, or importance of the impact of his love for an actual person on his poetry is debated, there is certainly no doubt that he is in love with the words of Love and with the forms in which these words are expressed. The powerful images he evokes, the clarity of expression, the passion he conjures up all point to a deliberate and carefully planned manipulation of the language. This is evident in the translations and I assume is even more apparent in the original.
Dante, too, seems to be in love with words, their power and their forms. If it is indeed true that the medieval writers and scholars were of a mind that "any word will do", then Petrarch's and Dante's departure from this model is quite clear. In addition to the clarity of thought and economy of words (by such I mean that words are carefully chosen not only for their meaning but for themselves, how well they fit, etc.) these writers, and Boccaccio express themselves from a very human angle. This is not to be confused with "humanity" as a general descriptive term but with "human" as in "individual." While Dante's theme of the progress of the human soul is generalizable, the perspective is very individualistic (Beatrice doesn't start the ball rolling for just anybody!). It is his fears, his reactions, his vision that we see. The same may be said for Petrarch's poems. While they can express universals they are written from the perspective of this individual. This is not just a difference in writing style from their predecessors but a real indication of the changing focus of their society. That a person can make contributions that are unique because of his individuality, that this individuality shapes and determines thought, and that it is important not only in and of itself but in the ways....what am I trying to express?! Perhaps an analogy....take the divine cosmos and pour it through a filter of humanity. Perhaps a medieval person would say the filter of one man will bring about the same results as the filter of another man, or rather we can see the results by using an Everyman filter. Dante, Petrarch, and Boccaccio would, I think, disagree. They would say, rather, that the Dante filter, the Petrarch filter, the Boccaccio filter will all produce different results. These results may be universalized but they are no less different and these differences are important.
Well this has degenerated into quite a mess. I wanted to also mention something about the selections all dealing with aspects of love: worldly or even earthy, ideal, and divine but perhaps this rambling has gone on quite long enough!
*Although Richard Levin in his commentary on Henry V (in a book whose name escapes me at the moment) has some interesting things to say on what he calls Fllewelynism, or this very human desire to draw comparisons and see similarities between even the most disparate things.